The Serious Fraud Office is considering whether to launch an investigation into the financial collapse of Royal Bank of Scotland, the banking group that has been rescued by the British government.

The SFO said it had been in contact with the Financial Services Authority to determine whether events (at RBS) were criminal or regulatory matters. "We are monitoring developments to ascertain whether or not there are circumstances warranting an intervention by the SFO," it said.

Although there is no formal criminal probe "at the moment," the SFO said it was looking into complaints made about RBS to the FSA and the Crown Office in Scotland, where the SFO has no formal remit.

At the weekend it was reported that Labour peer Lord Foulkes had been told that non-executive directors were intimidated by the way RBS was run by Sir Fred Goodwin, who left at the end of January with a £703,000-a-year pension.

The bank recently reported a £28bn loss, the largest in UK corporate history, which flowed from bad loan write-offs and losses on its controversial bid for Dutch bank ABN Amro at the height of the credit boom.

Calls for a full investigation into RBS have been strengthened by claims that its traders invested billions in so-called toxic debt, or sub-prime loans, as part of a tax avoidance operation. Though legal, the tax avoidance schemes were designed to exploit gaps in different countries' tax laws.

A department specialising in related "structured trades" has been closed down by new management, which has said that it would be wrong to take support from the Treasury on the one hand while seeking to avoid taxes on the other.

An RBS spokesman said: "The department has been shut in recent weeks and staff redeployed to other areas of our business."

The SFO said it would continue to keep developments at RBS under review at the same time as the FSA considered whether to launch a full-scale investigation into the way the bank was managed before the taxpayer took control last October.

The SFO said: "We have spoken on the phone to the FSA and other agencies about what is happening at RBS and will remain in touch for as long as is necessary."